1. Field of the Invention
This invention relates to a molten carbonate fuel cell, and in particular to a molten carbonate fuel cell having an improved collector plate.
2. Description of the Related Art
Various kinds of fuel cells have been proposed and commercially utilized. Among them, a molten carbonate fuel cell is now extensively studied and being developed because it is advantageous in power generation efficiency as well as it is capable of utilizing coal gas as a fuel gas.
This molten carbonate fuel cell is constructed such that a plurality of unit cell, each comprising an anode (a fuel electrode), a cathode (an air electrode) and an electrolyte body, are superimposed one upon another with a collector plate being interposed between the unit cells, and an interconnector is interposed between the collector plates to partition the unit cells. There is also disposed in the fuel cell a couple of edge-sealing plates for holding therebetween the peripheral portion of the electrolyte body. Thus, these edge-sealing plates are contacted to the peripheral portion of the electrolyte body so as to form a wet seal and to shield the unit cell from the outer atmosphere.
By the way, the collector plate disposed on the oxidizing gas-side is rendered to be covered with a film of highly corrosive molten carbonate solution which has been exuded from the air electrode during the operation of the fuel cell. Because of this, the collector plate is conventionally formed of stainless steel having an excellent corrosion resistance. The stainless steel conventionally employed is mainly SUS310S and SUS316L.
However, the collector plate formed of stainless steel is accompanied with a problem that a corrosion product containing corroded iron is deposited as a lamina on the surface of the collector plate during the operation of the fuel cell. In particular, when the fuel cell is operated for a long period of time extending over 1,000 hours, the deposition of corrosion product is grown up increasing the electric resistance thereof, thus extremely deteriorating the performance of the fuel cell. Namely, the deposition of the corrosion product on the collector plate deteriorates the performance and life of a fuel cell.
Japanese Patent Unexamined Publication No. 5-324460 discloses a stainless steel collector plate provided on the surface thereof with a NiO layer. According to this collector plate, the Ni layer is oxidized within a fuel cell thus forming a NiO layer, thereby preventing the deposition of a corrosive layer of high electric resistance, hence suppressing any increase in electric resistance. However, since the NiO layer of the collector plate is porous and attracts a large amount of electrolyte, the electrolyte-retention quantity within the unit cell may become insufficient thus possibly deteriorating the performance of the fuel cell. Moreover, a collector plate made of stainless steel or NiO-covered stainless steel may be deformed due to the influence of temperature distribution resulting from a difference in cooling rate of collector plate during cooling of thereof after power generation, hence the surface of the collector plate contacting with the cathode is peeled off thus spoiling the adhesion between the cathode and the collector plate. As a result, it is most probable that a molten carbonate fuel cell provided with a collector plate made of stainless steel or NiO-covered stainless steel may be suffering from deterioration in performance when it is subjected to a heat cycle wherein the temperature of collector plate is repeatedly raised or lowered.